Video Motion Detection (VMD) may be used for surveillance applications such as for perimeter protection and intrusion detection. It uses a computer to analyse a video sequence of the perimeter of a property, and provides notification which may include an alarm, if an intruder appears in the scene. Typically, such a system will operate in the near field, covering an area from a few meters in front of the camera to 30 meters away. The image has enough resolution to readily distinguish subjects in the near field, and barely sufficient resolution to distinguish objects at 30 meters or more.
To resolve objects that are further away, higher resolution cameras may be used to capture an image at a higher optical resolution. While the resolution of an image may be improved by software after the image has been captured, the end resolution is ultimately limited by the optical resolution at which the image is captured, which is a function of the camera optics and the image sensor that captures the image. For a given optical magnification of an image, an increase in optical resolution requires a more expensive image sensor, having an increase in sensor density and a corresponding increase in data. Similarly, an increase would occur if the end resolution were increased by software. The increase in data takes more processing to compress, send, and analyse. As a result, the rate at which video frames can be processed on modest hardware drops as the resolution increases. In security applications, since it is desirable to sense vehicles or fast running intruders that quickly cross the field of view; a high frame rate is desirable. This means that only low or medium resolution images can be analysed quickly enough on modest hardware to be useful in security applications.
Long range VMD can be effected by using a telephoto lens on the camera. However, the short range coverage is sacrificed to achieve this due to the narrow field of view of a telephoto lens. Furthermore, it is easy for an intruder in the foreground to obscure the view of the camera without the identity of the intruder or nature of the obscuration being evident from the video.
Long range Passive Infrared (PIR) devices have been developed to detect intruders over a range from a few meters to 150 meters away. PIR detectors have a pair of sensors that measure an intensity level of infrared radiation, and based on the shift in intensity level from one sensor to the other, determine whether moving infrared radiation is present and if an alarm condition is met. These detectors are very effective at detecting moving infrared radiation at the different ranges, but as the detection is of a pair of intensity levels only, the PIR does not know the shape of the object that causes a shift in intensity levels. Therefore, PIR alone can false alarm on animals such as foxes, rabbits and deer, and a typical installation requires a camera capable of zooming in on the area that causes an alarm to verify that the alarm condition is correct.
PIR detectors may be complemented with video cameras to allow video verification. Installing a PIR and a video camera with zoom capability as a complementary pair suffers from alignment issues since the PIR and the camera need to point at the same zones so that an operator can verify a PIR alarm quickly. In practice, it is difficult for an installer to align the two sensors. As a result, it is possible for a PIR alarm to be triggered in an area that the camera cannot see. U.S. Pat. No. 5,936,666 describes the collocation of two sensors in the same enclosure to overcome the issue of alignment of the fields of view of the two sensors during installation.
It is an object of the invention to at least in part alleviate one or more of the above-described limitations with known surveillance systems that rely on VMD and/or PIR devices.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.